The instant invention relates to power driven tools for reciprocating a cutting element such as a saw blade or the like. Such tools usually include some kind of shaft arrangement mounted in the gear case of the tool. The shaft arrangement will include a shaft slideably mounted in the gear case for reciprocating movement. The shaft is adapted to receive the cutting element thereon. A U-shaped channel yoke is mounted to the shaft for coacting with an eccentric drive driven by the motor of the power tool. The eccentric drive includes a pin and bearing which engages the U-shaped channel yoke and reciprocates the shaft arrangement during operation of the tool.
It is highly desirable for this shaft arrangement to have a minimal mass and yet be sufficiently strong that it can stand up well during the heavy-duty operations to which it is subjected when cutting work such as lumber, steel and the like.
A typical arrangement of the prior art includes an annular shaft having a notch-like cut-out formed therein for accommodating the U-shaped channel yoke. The manufacturing tolerances of the notch-like cut-out in the shaft and of the channel yoke have to be kept within defined limits so that a reliable brazed joint is obtained between the U-shaped channel yoke and the shaft. A good brazed joint is essential in this arrangement because brazed joint must transmit the entire reciprocating drive force from the yoke to the shaft. Maintaining the close tolerances for the yoke has proven in practice to be very expensive because of relatively large material thickness and forming tolerances. For example, a small bend radius has to be provided for the U-shaped channel yoke where the leg portions of the yoke join the base of the yoke. Otherwise, the notch-like cut-out in the shaft will have to be of excessive depth thereby weakening the shaft.
Also, it has been shown that a small bend radius in a channel yoke contributes to weakness of the yoke. Whereas, a large bending radius is desirable because the bending operation performed to manufacture the U-shaped channel yoke will not cause adverse stress concentrations in the material and not appreciably weaken the channel yoke. Although extrusion of steel would appear to afford a possible means of manufacturing a channel yoke with a narrow bend radius, it has been impractical to do so for the dimensions required in the channel yoke of a reciprocating power tool.
In addition, in order to hold the parts together during brazing, a rivet is often utilized to hold the yoke to the shaft as the brazing joint is formed. The hole made in the shaft for accommodating the rivet further weakens the shaft at the exact point where the shaft has already been weakened by the cut-out required to accommodate the channel yoke.
The above requirements make it necessary to use a shaft of heavy material to afford the necessary strength needed to compensate for the weakened structure at the yoke-shaft interface. This add to the discomfort of the operator because a larger mass must be reciprocated by the eccentric drive thereby imparting vibrations to the housing of the tool and to the tool operator.
The prior art shaft arrangement also required a clean up operation after the brazing step had been performed. Brazing material will inevitably run from the joint region over the surface of the shaft. Since the shaft wall constitutes a bearing surface it must be clean and well polished. Accordingly a clean-up step for removing excessive brazing must also be performed.